Let me get this right.., 75 dB -> 22.5W, 78 dB-> 45 W, 81 dB -> 90W , 84 dB -> 180 W . The 10x (of 225W) for reference level is an approximation ?
My numbers are
exact!! Your starting point of 75 dB -> 22.5W is incorrect. From the data you provided, I assume your actual average dB measured was 75 dB, and assuming you had two speakers on. May be I should have assumed just 1 speaker to error on the conservative side.
Let's start again, from the calculator, it should be 75 dB -> 0.225 W
Therefore: 78 dB-> 0.45 W, 81 dB -> 0.9 W , 84 dB -> 1.8 W, and 85 dB -> 2.25, 95 dB -> 22.5 W
That is, for 75 dB average, allowing for 20 dB peak, the calculator would tell you 22.5 WPC will do the job and it comes with a build in 3 dB reserve as the calculator is using peak wattage values and 22.5 W average has a peak of 45 W peak (that's 3 dB).
Now back to your initial 75 dB average that you seemingly need, the calculator shows you need only 2.25 W average. Your need at least 100 X that to ensure your amp doesn't clip on those rare 20 dB peak short moments. 100X0.225 = 22.5 WPC. 10X ->10 dB, and 20X ->20 dB
If you were to target the so called THX ref level, then you need another 10 dB increase, and you power requirement would therefore rise another 10X, that is 225 WPC.
You can see why the how much power do I need and do I need an external amp is such an loaded question.. Every 2X the power output only get you 3 dB more SPL, and for 10 dB more you need 10X the output.
So you'd still favor a X3400(or 3600h) or even X4500( with price reduced) over something like Marantz PM7000N or Denon PMA-1600NE ? I do like the compacts in my office , but the prices are not attractive..
I have no use for those PM and PMA models, no idea why they even bothered.. The AVR-X3600H is such as great deal right now but too bad only in Canada. In the US, I would wait for the SR7013 or X4500H to drop below the $999 mark. It's been there before so I firmly believe it will be back.
Also, saw this note on NAD support for Airplay2 on certain models
Airplay2 support on NAD devices. One thing I noticed on these NAD amps is the lower distortion on 4 ohms.. , while the D&M amps have atleast 0.7% THD at 4Ohms with 2 channels driven.
I don't know which NAD devices you referred to. If you look at the ranking over at ASR, the T758 V3's THD+N currently ranked at the bottom of the chart, the T777 did a little better, still rank below D&M's iirc. For those brands that often get promoted by their fans (I guess they paid more relatively speaking so easy for them to hear better sound quality..) such as NAD and Anthem, you practically have pay more for less.